The present invention relates to pressure transmitters used to measure fluid pressures in industrial fluid processing plants. In particular, the present invention relates to the thermal performance of such pressure transmitters.
Pressure transmitters are known to have undesired output changes when there are changes in process fluid temperature, ambient air temperature or even changes in temperature resulting from rainfall on the transmitter housings. Warm-up errors due to localized internal power dissipation are especially difficult to correct under both transient and steady state thermal conditions.
In high accuracy pressure transmitters, these output changes are controlled electronically by compensating temperature and mechanically by using two-part transmitter housings with relatively large surface areas that are joined with a threaded neck. The threaded neck separates and thermally isolates the pressure sensor and the fluid isolator from the heat generated in a circuit controlling a current output.
High accuracy circuits, sensors, and isolators are becoming more miniaturized. There is a desire for a correspondingly miniaturized, high accuracy transmitter in a single housing without the need for a threaded neck to separate the heat generated by the circuit from the pressure sensor and fluid isolator.
It is difficult, however, to manage thermal problems in such miniature housings where there is less housing surface area to dissipate heat. Problems also arise because the sensor and isolator are much closer to the heat dissipating output circuit components. A method and apparatus are needed that will provide a high accuracy, temperature compensated pressure transmitter in a single miniaturized housing.
A pressure transmitter in accordance with the present invention includes a miniature transmitter housing with an outer wall providing thermal resistance. The outer wall separates an isothermal island at a first end of the housing from a heat sink thermal island at a second end of the housing.
The isothermal island includes a fluid inlet, a pressure sensor for detecting fluid pressure from the inlet, and a temperature sensor.
The heat sink thermal island includes an electrical connector with an outer shell and a feedthrough with contacts. The heat sink island also includes a primary heat load device that is heat sunk to the outer shell. The contacts carry a current controlled by the transmitter. A major portion of that current is conducted by the primary heat load device under normal operating conditions.
The pressure transmitter controls the current to provide a temperature-compensated, high accuracy representation of the pressure.